Walk Off the Earth Singer Sarah Blackwood -- United Airlines Booted Me 'Cause My Kid Was Crying

United Airlines has a problem with crying infants according to alternative rocker Sarah Blackwood ... who says she was kicked off a flight because her 1-year-old son got fussy -- but the airline claims the kid was a safety risk.

Sarah -- Walk Off the Earth singer -- is 7 months pregnant and says UA kicked her off the San Fran to Vancouver flight ... after less than 10 minutes of Giorgio squirming and crying. She says he fell asleep after the fit, but for the airline the damage was done. Sarah says the plane stopped taxiing and returned her to the terminal.

Sarah's fellow passengers saw her and her son get escorted off the plane -- and many of them took to social media to express their outrage over the situation. Let's face it ... babies cry on planes all the time.

We contacted United and a rep says the flight crew made a difficult decision, but made the correct one “based solely on safety concerns." The rep claims, "the child was not seated, as required by federal regulation to ensure passenger safety, and was repeatedly in the aisle of the aircraft before departure and during taxi."

Sarah's calling BS on that. On TMZ Live, she told us she was in a window seat and her fellow passengers can vouch for her that Giorgio was not cruising the aisles.

Sarah also recorded a sympathetic United employee ... who suggested she use the power of social media to put pressure on the airline.